In mobile communication business, an amount of traffic increases rapidly according to spread of a cellular phone and service diversification thereof. Under such background, a network in which mobile base stations (i.e. the number of cells) increases and a distance between cells is configured. In a microwave communication network which connects mobile base stations with each other, a link in which a transmission distance is short is useful and a large number of frequency channels are used. Accordingly, in the microwave communication network, a high frequency band of no smaller than 10 GHz is mainly used.
Generally, the microwave communication network suffers from degradation of line quality (occurrence of a line failure) caused by fading and reflection. In order to improve the degradation, technique such as frequency diversity and space diversity is applied. However, in communication between the mobile base stations in which the high frequency band of no smaller than 10 GHz is used, the degradation of the line quality caused by rainfall is larger than that caused by the fading and the reflection. Accordingly, the degradation of the line quality (occurrence of the line failure) caused by the rainfall can not be avoided even through a communication method or a system configuration represented by the frequency diversity or the space diversity is used. Consequently, large amount of rainfall causes line disconnect.
Further, in this specification, the line failure is defined to be a generic designation of a failure which occurs in the line, that is, the line disconnect, instantaneous line disconnect thereof or the like.
It is very serious problem that a large amount of rainfall leads to the line failure and the line disconnect as mentioned above with respect to maintaining the line quality and providing a high-value added service.
Patent document 1 (Japanese Patent Application Laid-Open No. 2005-268977) discloses a radio base station which can detect an actual cluster of water vapor/rain cloud in FIG. 3 and FIG. 5 thereof. In the document 1, the radio base station uses an effect that a navigation signal transmitted from GPS (Global Positioning System) satellite is received by the radio base station with a certain delay caused by existence of the actual cluster of water vapor/rain cloud. Moreover, the patent document 1 discloses in FIG. 3 thereof that when the radio base station judges that there exists the actual cluster of water vapor/rain cloud by detecting the delay on receiving the navigation signal from the GPS satellite, the radio base station automatically adjusts a transmitting power/an failure correcting capability and prevents disconnect of a wireless line caused by the rainfall. Furthermore, the patent document 1 discloses in FIG. 5 thereof that the whole network shares information on existence of the actual cluster of water vapor/rain cloud which the radio base station obtains by detecting the delay of the received navigation signal from the GPS satellite, a route different from the route suffering from the disconnect of the wireless line caused by the rainfall is set up on the basis of the shared information, and consequently the network avoids the disconnect of the wireless line caused by the rainfall.
Patent document 2 (Japanese Patent Application Laid-Open No. 2006-203793) discloses in FIG. 1 thereof that a satellite communication station detects transmission loss caused by actual rainfall or the like by detecting that an attenuation amount of a received signal level is not smaller than a predetermined value. Moreover, the patent document 2 discloses in FIG. 1 thereof that when the satellite communication station detects the transmission loss caused by the actual rainfall or the like by detecting that an attenuation amount of a received signal level is not smaller than a predetermined value, the satellite communication station transmits a line change request signal to a control station. Moreover, the patent document 2 discloses that when the control station receives the line change request signal, the control station re-assigns a communication line by changing a transmission frequency of the satellite communication station, reducing a transmission frequency bandwidth and increasing transmission level.
Patent document 3 (Japanese Patent Application Laid-Open No. 2007-47148) discloses a navigation system which includes a navigation apparatus and an information center as shown in FIG. 1 thereof, and whose information center includes a means for forecasting weather at a predicted arrival time in different places on a traveling road on the basis of a weather forecast in different places to travel. However, the weather forecast (sunny, cloudy, rainy or the like) at the predicted arrival time in different places to travel (as described in the paragraph [0062] of the patent document 3) is only used to search for a predicted average traveling time according to the weather forecast along the road.
Patent document 4 (Japanese Patent Application Laid-Open No. 2007-37029) discloses a radio communication system which adopts an adaptive modulation method. A transmission apparatus of the radio communication system also includes a rainfall detecting unit and changes modulation method according to an amount of rainfall detected by the rainfall detecting unit. The line failure may occur according to extent of the amount of rainfall, since an actual rainfall during detecting the actual rainfall causes degradation of line quality.
As mentioned above, an apparatus as a network constituent such as the radio base station in the patent document 1, the satellite communication station in the patent document 2 and the transmission apparatus in the patent document 4 includes the rainfall detecting unit and judges degradation of the line quality after it begins to rain actually. If degradation of the line quality is judged after detecting the actual rainfall, the most rainfall except for a light rainfall (that is, usual rainfall, heavy rainfall, squall or the like) may cause the line failure or the line disconnect at the point of time on detecting the actual rainfall. Moreover, there is a problem that a temporary line disconnect may occur before changing line route to a different line route or before changing modulation method.